The present invention relates to a system which minimizes the transfer of vibration forces and moments from a vibrating body to a body attached thereto.
Vibration in helicopters causes many undesirable effects. These include: crew fatigue, resulting in decreased proficiency; unacceptable passenger discomfort; decreased component reliability, resulting in increased operating costs; and, in many cases, limited maximum cruising speed.
The main rotor-transmission assembly (the "pylon") is a major source of helicopter vibration. In operation, the rotor causes pylon vibration in all six degrees of freedom; that is, vertical, lateral, and longitudinal forces, and roll, pitch, and yaw moments. The predominant pylon vibration harmonic occurs at the blade passage frequency (the "b/rev frequency"), which is equal to the number of rotor blades times the angular velocity of the rotor.
Early pylon mounting systems resulted in fuselage vibration levels which exceeded 0.5 g. Next, isolation systems using focal isolation mounts were generally able to limit b/rev fuselage vibration at cruise airspeeds to about 0.15 g, but vibration at transition airspeeds (approximately 0 to 25 knots) exceeded that level.
During the 1970's, the U.S. military reduced the b/rev vibration standard to 0.05 g at cruise airspeeds. Several pylon isolation systems which included one or more antiresonant, force-cancelling devices were developed in an attempt to meet that standard. Each of the force-cancelling devices included a spring and a mechanically-amplified tuning mass to partially or completely cancel pylon b/rev vibration in one degree of freedom. Systems combining one or more such force-cancelling devices with focal mounts were able to effect at least partial isolation in up to five degrees of freedom. However, such systems were complex, expensive, required considerable space, and imposed a weight penalty which varied from 2-3% of helicopter design weight; and none of the systems was able to meet the 0.05 g standard.
U.S. Pat. No. 4,236,607 (Halwes et al.) discloses a spring-tuning mass vibration isolator in which force cancellation is accomplished by hydraulically amplifying the inertia of a liquid tuning mass. The Halwes Liquid Inertial Vibration Eliminator ("LIVE") isolator 1 is shown schematically in cross section in FIG. 1. A layer of low-damped rubber 3 is bonded between the outer surface of a piston 5 and the inner surface of a cylinder 7. The rubber 3 acts as an elastomeric spring and as a liquid seal. Upper and lower end caps 9, 11, respectively, are secured to the ends of the cylinder 5 to prevent fluid leakage from the interior thereof, thereby forming two chambers 13, 15. The chambers 13, 15 are connected by a tuning passage 17 in the piston 5, and the chambers 13, 15 and the tuning passage 17 are filled with a high-density, incompressible, low-viscosity fluid such as, for example, liquid mercury.
A vibrating body 19 is attached to the upper end cap 9. The vibrating body oscillates in the direction indicated by arrow 21. A body 23 to be isolated from the vibration (the "isolated body") of the vibrating body 19 is connected to the piston 5 by means of a bracket 25 and lugs 27. The oscillatory force produced by the vibrating body 19 in the direction 21 causes relative motion between the piston 5 and the cylinder 7. That relative motion creates an oscillatory reaction force due to strain in the rubber spring 3. At the same time, the volumes of the chambers 13, 15 are alternately increased and decreased, and the liquid contained in the chambers 13, 15 and the tuning passage 17 is pumped back and forth between the chambers 13, 15 through the tuning passage 17. The inertial mass of the liquid in the tuning passage 17 (the "tuning mass") is amplified by the ratio of the effective cross-sectional area of the piston 5 and rubber spring 3 (the "effective piston area") to the cross-sectional area of the tuning passage 17 (the "tuning passage area"). The inertial force created by acceleration of the tuning mass is out of phase with the reaction force of the rubber spring 3. In a system with no damping, at some frequency (the "isolation frequency"), the inertial force becomes equal and opposite to the spring force, complete force cancellation occurs, and no vibration is transferred to the isolated body 23. In a system having damping, complete force cancellation does not occur, but minimum transfer of vibration to the isolated body occurs at the isolation frequency. The isolation frequency for the vibrating body-LIVE isolator-isolated body system, f.sub.I, is calculated as follows: ##EQU1## k=the spring rate of the rubber spring; R=the ratio of the effective piston area to the tuning passage area;
L=the length of the tuning passage; PA1 A=the tuning passage area; and PA1 .rho.=the mass density of the liquid.
As the isolation effect of the LIVE isolator is dependent on the inertial effect caused by pumping the liquid, the LIVE isolator is effective only along an axis which is perpendicular to, and passes through the geometric center of, the effective piston area (the "operating axis"). Thus, when a vibration force is applied to a LIVE isolator 1 along other than the isolator's operating axis, only the component of the vibration force along the operating axis is isolated.
Curve 27 in FIG. 2 shows a plot of the frequency response of the isolated body 25 along the operating axis of the LIVE oscillator 1. The curve 27 is for a system having approximately one percent critical damping. Line 29 represents the response of the isolated body 25 along the operating axis for an equivalent rigid-body system. As can be seen, a relatively narrow isolation "notch" 30 in the curve 27 occurs in the vicinity of the isolation frequency, f.sub.I (also known as the antiresonance frequency). Maximum isolation (minimum isolated body response) is 99% at the isolation frequency; that is, only 1% of the vibrating body force is transferred to the isolated body at the isolation frequency.
The Halwes LIVE isolator 1 is rugged, compact, lightweight, self-contained, and provides excellent vibration isolation along its operating axis for vibration near the isolation frequency of the vibrating body-LIVE isolator-isolated body system.
Other inertial isolators are known in the art. See, for example, U.S. Pat. No. 4,811,919 (Jones) and U.S. Pat. No. 5,174,552 (Hodgson et al.), each of which show inertial isolators having an external tuning passage. However, in each inertial isolator other that the Halwes LIVE isolator, the configuration of the tuning passage is such that it presents a greater resistance to liquid flow than in the LIVE isolator. That flow resistance increases the damping of the isolator, which decreases the isolator's effectiveness.
D. R. Halwes, "Total Main Rotor Isolation System," Bell Helicopter Textron Inc. (1981) and D. R. Halwes, "Controlling the Dynamic Environment During NOE Flight," Bell Helicopter Textron Inc. (1985) describes a six degree of freedom ("6 DOF") helicopter pylon isolation system. The system uses six LIVE isolator links to attach the pylon to the fuselage and to isolate b/rev vibration in all six degrees of freedom.
Although the 6 DOF system provides b/rev isolation that is superior to that of previous systems, it has several shortcomings. First, to ensure that each LIVE isolator is exposed only to forces along its operating axis, each LIVE isolator link is pinned at each of its ends by means of elastomeric bearings. In addition to being expensive, the elastomeric bearings introduce additional damping into the pylon-LIVE isolator-fuselage system, which decreases the effectiveness of the isolators.
Second, since the effectiveness of a LIVE isolator decreases markedly as isolation frequency varies from the vibration frequency, relatively minor changes in the 6 DOF system which effect the isolation frequency produce a relatively large decrease in the system's effectiveness. Such changes include: changes in the isolators and elastomeric bearings due to aging; changes in the properties of the rubber springs, elastomeric bearings, and liquid due to the temperature variations; and variations between the system's isolators due to manufacturing tolerances. In addition, a helicopter's main rotor is often operated at other than its nominal rotational speed, resulting in a mismatch between the main rotor b/rev frequency and the 6 DOF system's isolation frequency, which decreases the isolators' effectiveness. A means for adjusting the 6 DOF system's isolators during operation to compensate for such changes and to match the system's isolation frequency to the main rotor b/rev frequency would allow the system to provide optimal b/rev isolation at all times.
The isolation frequency of a system which includes a LIVE or other inertial isolator can be changed by changing the length or the cross-sectional area of the isolator's inertial passage. For example, see U.S. Pat. No. 4,969,632 (Hodgson et al.) and U.S. Pat. No. 4,641,808 (Flower). However, the prior art means for effecting such changes significantly increase isolator damping, which decreases the isolator's effectiveness. A means for changing isolator inertial passage length or cross-sectional area without significantly increasing damping would allow the system's isolation frequency to be changed while maintaining high isolator effectiveness.
Third, the 6 DOF system is relatively space-consuming. A more compact system would save both space and weight.